a) Field of the Invention
The invention is directed to a process for the production of superconductor molded bodies from rare-earth transition-metal boron carbide and boron nitride compounds.
b) Description of the Prior Art
Rare-earth transition-metal boron carbide and boron nitride compounds with superconducting characteristics by which transition temperatures of up to 23 K are achieved are already known. In this respect, representatives of the SE--Ni--B--C system (SE=Y, Tm, Er, Ho, Lu, Dy) have been thoroughly investigated heretofore, where SE Ni.sub.2 B.sub.2 C was identified as a superconducting phase (R. J. Cava, et al., Nature 367, 252 (1994)).
Other superconducting phases with transition temperatures of 6 to 15 K were found for the compounds SE Ni.sub.4 B.sub.4 C (SE =Y, Tm, Er, Ho), for YNi.sub.4 BC.sub.0,2, for YNi.sub.2 B.sub.3 C.sub.0,2 and for La.sub.3 Ni.sub.2 BN.sub.3 (Q. W. Yan, et al., Phys. Rev. B 51, 8395 (1995); R. Nagarajan, et al., Phys. Lett. 72, 274 (1994); R. J. Cava, et al., Nature 372, 245 (1994); C. Godart, et al., J. Solid State Chemistry 133,169 (1997)). Even without further material optimization, the transition temperatures are accordingly in the range of the classic A15 superconductor.
It is known to use melt-metallurgical methods for the production of rare-earth transition-metal boron carbide and boron nitride compounds for superconductors. In so doing, the high melting temperatures of the intermetallic boron carbide and boron nitride superconductors require preparation by means of light-arc or levitation smelting techniques. Heretofore, primarily polycrystalline specimens were produced in this way (for example: M. Buchgeister, et al., Mat. Lett. 22, 203 (1995); R. J. Cava, et al., Nature 372, 245 (1994); C. Godart, et al., J. Solid State Chemistry 133, 169 (1997)). Further, corresponding monocrystals of intermetallic boron carbide superconductors were also prepared (for example: T. Grigereit, et al., Phys. Rev. Lett. 73, 2756 (1995)).
As a result of selective evaporation of individual components, melt-metallurgical production leads to problems with respect to the required precise adjustment of the stoichiometry and in the production of phase-pure materials (M. Buchgeister, et al., Mat. Lett. 22, 203 (1995)). On the other hand, the phase formation is determined by the thermodynamics within the framework of the equilibrium phase diagrams. Thus, for example, in the Y--Pd--B--C system with melt-metallurgical production, while a high transition temperature of 23 K is achieved, it has so far not been possible to isolate the corresponding superconductor phase in a pure phase (Y. Y. Sun, et al., Physica C 230, 435 (1994)).
It is also known to produce strip-shaped boron carbide superconductors with the nominal composition YNi.sub.2 B.sub.2 C by means of the process of rapid solidification, wherein a partial amorphizing occurs during solidification (V. Strom, et al., Physica C 235-240, 2537 (1994)). Amorphizing means that the amorphous phase has a glasslike, non-crystalline structure without translational crystallographic symmetry as it is well-established for a variety of metallic alloys also including metallic alloys with elements such as Y, SE elements, transition metals as well as carbon or nitrogen (for example: L. Schultz, J. Eckert, in "Glassy Metals III", Eds. H. Beck, H.-J. Guintherodt, Springer Verlag, Berlin (1994), p. 69; H. Koshiba, et al., Nanostructured Mater. 8, 997 (1997). In this case, the achieved YNi.sub.2 B.sub.2 C crystallites are presumably present in a boron-enriched amorphous matrix. Compact molded bodies cannot be produced by the technique of rapid solidification.
Finally, it is known to generate boron carbide superconductors by means of sputtering thin films (S. Arisawa, et al., Appl. Phys. Lett., 65, 1299 (1994)). In this case, amorphous Y--Pd--B--C films with a thickness of 300 nm were generated by deposition on Mg--O substrates at room temperature. The amorphous phase taking place during the deposition, is changed into the superconducting YNi.sub.2 B.sub.2 C phase by subsequent annealing at 1050.degree. C. The crystallization temperature is accordingly disadvantageously in the range of those temperatures typically used in melt-metallurgical production. Specimens that are annealed below this temperature, e.g., at 900.degree. C., do not exhibit superconducting properties. The production of compact molded bodies is also impossible with this technique.